SYNOPSIS

VERSION

DESCRIPTION

This document serves to answer the most frequently asked questions on both
the DBI Mailing Lists and personally to members of the DBI development team.

Basic Information & Information Sources

1.1 What is DBI, DBperl, Oraperl and *perl?

To quote Tim Bunce, the architect and author of DBI:

``DBI is a database access Application Programming Interface (API)
for the Perl Language. The DBI API Specification defines a set
of functions, variables and conventions that provide a consistent
database interface independant of the actual database being used.''

In simple language, the DBI interface allows users to access multiple database
types transparently. So, if you connecting to an Oracle, Informix, mSQL, Sybase
or whatever database, you don't need to know the underlying mechanics of the
3GL layer. The API defined by DBI will work on all these database types.

A similar benefit is gained by the ability to connect to two different
databases of different vendor within the one perl script, ie, I want
to read data from an Oracle database and insert it back into an Informix
database all within one program. The DBI layer allows you to do this simply
and powerfully.

DBperl is the old name for the interface specification. It's usually
now used to denote perl4 modules on database interfacing, such as,
oraperl, isqlperl, ingperl and so on. These interfaces
didn't have a standard API and are generally not supported.

Here's a list of DBperl modules, their corresponding DBI counterparts and
support information. Please note, the author's listed here generally
do not maintain the DBI module for the same database. These email
addresses are unverified and should only be used for queries concerning the
perl4 modules listed below. DBI driver queries should be directed to the
dbi-users mailing list.

However, some DBI modules have DBperl emulation layers, so, DBD::Oracle
comes with an Oraperl emulation layer, which allows you to run legacy oraperl
scripts without modification. The emulation layer translates the oraperl API
calls into DBI calls and executes them through the DBI switch.

The Msqlperl emulation is a special case. Msqlperl is a perl5 driver
for mSQL databases, but does not conform to the DBI Specification. It's
use is being deprecated in favour of DBD::mSQL. Msqlperl may be downloaded
from CPANvia:

1.2. Where can I get it from?

The Comprehensive Perl Archive Network
resources should be used for retrieving up-to-date versions of the DBI
and drivers. CPAN may be accessed via Tom Christiansen's splendid
CPAN multiplexer program located at:

PODs are chunks of documentation usually embedded within perl programs
that document the code ``in place'', providing a useful resource for
programmers and users of modules. POD for DBI and drivers is beginning to
become more commonplace, and documentation for these modules can be read
with the "perldoc" program included with Perl.

The DBI Specification

The POD for the DBI Specification can be read with the:

perldoc DBI

command. The Specification also forms Appendix A of ``Programming the Perl
DBI''.

Oraperl

Users of the Oraperl emulation layer bundled with DBD::Oracle, may read
up on how to program with the Oraperl interface by typing:

perldoc Oraperl

This will produce an updated copy of the original oraperl man page written by
Kevin Stock for perl4. The oraperl API is fully listed and described there.

Drivers

Users of the DBD modules may read about some of the private functions
and quirks of that driver by typing:

perldoc <driver>

For example, the DBD::mSQL driver is bundled with driver-specific
documentation that can be accessed by typing

perldoc DBD::mSQL

Frequently Asked Questions

This document, the Frequently Asked Questions is also available as POD
documentation! You can read this on your own system by typing:

perldoc DBI::FAQ

This may be more convenient to persons not permanently, or conveniently,
connected to the Internet. The DBI::FAQ module should be downloaded and
installed for the more up-to-date version.

The version of DBI::FAQ shipped with the "DBI" module may be slightly out
of date.

POD in general

Information on writing POD, and on the philosophy of POD in general, can be
read by typing:

perldoc perlpod

Users with the Tk module installed may be interested to learn there is a
Tk-based POD reader available called "tkpod", which formats POD in a convenient
and readable way. This is available viaCPAN as the module called
Tk::POD and is highly recommended.

Driver and Database Characteristics

The driver summaries that were produced for Appendix B of ``Programming the
Perl DBI'' are available online at:

in the driver information table. These summaries contain standardised
information on each driver and database which should aid you in selecting
a database to use. It will also inform you quickly of any issues within
drivers or whether a driver is not fully compliant with the DBI Specification.

There are a series of occasional rambles from various people on the
DBI mailing lists who, in an attempt to clear up a simple point, end up
drafting fairly comprehensive documents. These are quite often varying in
quality, but do provide some insights into the workings of the interfaces.

Articles

A list of articles discussing the DBI can be found on the DBIWWW page at:

These articles are of varying quality and age, from the original Perl Journal
article written by Alligator and Tim, to more recent debacles published online
from about.com.

README files

The README files included with each driver occasionally contains
some useful information ( no, really! ) that may be pertinent to the user.
Please read them. It makes our worthless existences more bearable. These
can all be read from the main DBIWWW page at:

Compilation Problems

2.1. Compilation problems or It fails the test!

First off, consult the README for that driver in case there is useful
information about the problem. It may be a known problem for your given
architecture and operating system or database. You can check the README
files for each driver in advance online at:

Please email the address listed in the WWW pages for whichever driver you
are having problems with. Do not directly email the author at a
known address unless it corresponds with the one listed.

We tend to have real jobs to do, and we do read the mailing lists for
problems. Besides, we may not have access to <insert your
favourite brain-damaged platform here> and couldn't be of any
assistance anyway! Apologies for sounding harsh, but that's the way of it!

However, you might catch one of these creative genii at 3am when we're
doing this sort of stuff anyway, and get a patch within 5 minutes. The
atmosphere in the DBI circle is that we do appreciate the users'
problems, since we work in similar environments.

If you are planning to email the author, please furnish as much information
as possible, ie:

•

ALL the information asked for in the README file in
the problematic module. And we mean ALL of it. We don't
put lines like that in documentation for the good of our health, or
to meet obscure README file standards of length.

•

If you have a core dump, try the Devel::CoreStack module for
generating a stack trace from the core dump. Send us that too.
Devel::CoreStack can be found on CPAN at:

Remember, the more information you send us, the quicker we can track
problems down. If you send us no useful information, expect nothing back.

Finally, please be aware that some authors, including Tim Bunce, specifically
request that you do not mail them directly. Please respect their wishes and
use the email addresses listed in the appropriate module "README" file.

Email the dbi-users Mailing List

It's usually a fairly intelligent idea to cc the mailing list
anyway with problems. The authors all read the lists, so you lose nothing
by mailing there.

Platform and Driver Issues

3.1 What's the difference between ODBC and DBI?

In terms of architecture - not much: Both define programming
interfaces. Both allow multiple drivers to be loaded to do the
actual work.

In terms of ease of use - much: The DBI is a 'high level' interface
that, like Perl itself, strives to make the simple things easy while
still making the hard things possible. The ODBC is a 'low level'
interface. All nuts-bolts-knobs-and-dials.

Now there's an ODBC driver for the DBI (DBD::ODBC) the ``What's the
difference'' question is more usefully rephrased as:

Chapter 7 of ``Programming the Perl DBI'' covers this topic in far more
detail and should be consulted.

3.2 What's the difference between Win32::ODBC and DBD::ODBC?

The DBI, and thus DBD::ODBC, has a different philosophy from the
Win32::ODBC module:

The Win32::ODBC module is a 'thin' layer over the low-level ODBCAPI.
The DBI defines a simpler 'higher level' interface.

The Win32::ODBC module gives you access to more of the ODBCAPI.
The DBI and DBD::ODBC give you access to only the essentials.
(But, unlike Win32::ODBC, the DBI and DBD::ODBC do support parameter
binding and multiple prepared statements which reduces the load on
the database server and can dramatically increase performance.)

The Win32::ODBC module only works on Win32 systems.
The DBI and DBD::ODBC are very portable and work on Win32 and Unix.

The DBI and DBD::ODBC modules are supplied as a standard part of the
Perl 5.004 binary distribution for Win32 (they don't work with the
older, non-standard, ActiveState port).

Scripts written with the DBI and DBD::ODBC are faster than Win32::ODBC
on Win32 and are trivially portable to other supported database types.

The DBI offers optional automatic printing or die()ing on errors which
makes applications simpler and more robust.

The current DBD::ODBC driver version 0.16 is new and not yet fully stable.
A new release is due soon [relative to the date of the next TPJ issue :-]
and will be much improved and offer more ODBC functionality.

To summarise: The Win32::ODBC module is your best choice if you need
access to more of the ODBCAPI than the DBI gives you. Otherwise, the
DBI and DBD::ODBC combination may be your best bet.

Chapter 7 of ``Programming the Perl DBI'' covers this topic in far more
detail and should be consulted.

3.3 Is DBI supported under Windows 95 / NT platforms?

Finally, yes! Jeff Urlwin has been working diligently on building
DBI and DBD::ODBC under these platforms, and, with the
advent of a stabler perl and a port of MakeMaker, the project has
come on by great leaps and bounds.

The DBI and DBD::Oracle Win32 ports are now a standard part of DBI,
so, downloading DBI of version higher than 0.81 should work fine as
should using the most recent DBD::Oracle version.

3.4 Can I access Microsoft Access or SQL-Server databases with DBI?

3.5 Is the a DBD for <insert favourite database here>?

If not, no. A complete absence of a given database driver from that
page means that no-one has announced any intention to work on it, not that
such a driver is impossible to write.

A corollary of the above statement implies that if you see an announcement
for a driver not on the above page, there's a good chance it's not
actually a DBI driver, and may not conform to the specifications. Therefore,
questions concerning problems with that code should not really be addressed
to the DBI Mailing Lists.

3.6 What's DBM? And why should I use DBI instead?

Extracted from ``DBI - The Database Interface for Perl 5'':

``UNIX was originally blessed with simple file-based ``databases'', namely
the dbm system. dbm lets you store data in files, and retrieve
that data quickly. However, it also has serious drawbacks.
File Locking
The dbm systems did not allow particularly robust file locking
capabilities, nor any capability for correcting problems arising through
simultaneous writes [ to the database ].
Arbitrary Data Structures
The dbm systems only allows a single fixed data structure:
key-value pairs. That value could be a complex object, such as a
[ C ] struct, but the key had to be unique. This was a large
limitation on the usefulness of dbm systems.
However, dbm systems still provide a useful function for users with
simple datasets and limited resources, since they are fast, robust and
extremely well-tested. Perl modules to access dbm systems have now
been integrated into the core Perl distribution via the
AnyDBM_File module.''

To sum up, DBM is a perfectly satisfactory solution for essentially read-only
databases, or small and simple datasets. However, for more
scaleable dataset handling, not to mention robust transactional locking,
users are recommended to use a more powerful database engine viaDBI.

3.7 What database do you recommend me using?

This is a particularly thorny area in which an objective answer is difficult
to come by, since each dataset, proposed usage and system configuration
differs from person to person.

From the current author's point of view, if the dataset is relatively
small, being tables of less than 1 million rows, and less than 1000 tables
in a given database, then mSQL is a perfectly acceptable solution
to your problem. This database is extremely cheap, is wonderfully robust
and has excellent support. More information is available on the Hughes
Technology WWW site at:

If the dataset is larger than 1 million row tables or 1000 tables, or if you
have either more money, or larger machines, I would recommend Oracle RDBMS.
Oracle's WWW site is an excellent source of more information.

Informix is another high-end RDBMS that is worth considering. There are
several differences between Oracle and Informix which are too complex for
this document to detail. Information on Informix can be found on their
WWW site at:

In the case of WWW fronted applications, mSQL may be a better option
due to slow connection times between a CGI script and the Oracle RDBMS and
also the amount of resource each Oracle connection will consume. mSQL
is lighter resource-wise and faster.

These views are not necessarily representative of anyone else's opinions,
and do not reflect any corporate sponsorship or views. They are provided
as-is.

3.8 Is <insert feature here> supported in DBI?

Given that we're making the assumption that the feature you have requested
is a non-standard database-specific feature, then the answer will be no.

DBI reflects a genericAPI that will work for most databases, and has
no database-specific functionality.

However, driver authors may, if they so desire, include hooks to database-specific
functionality through the "func()" method defined in the DBIAPI.
Script developers should note that use of functionality provided via
the "func()" methods is very unlikely to be portable across databases.

Programming Questions

4.1 Is DBI any use for CGI programming?

In a word, yes! DBI is hugely useful for CGI programming! In fact, I would
tentatively say that CGI programming is one of two top uses for DBI.

DBI confers the ability to CGI programmers to power WWW-fronted databases
to their users, which provides users with vast quantities of ordered
data to play with. DBI also provides the possibility that, if a site is
receiving far too much traffic than their database server can cope with, they
can upgrade the database server behind the scenes with no alterations to
the CGI scripts.

4.2 How do I get faster connection times with DBD::Oracle and CGI?

Contributed by John D. Groenveld

The Apache "httpd" maintains a pool of "httpd" children to service client
requests.

Using the Apache mod_perl module by Doug MacEachern, the perl
interpreter is embedded with the "httpd" children. The CGI, DBI, and your
other favorite modules can be loaded at the startup of each child. These
modules will not be reloaded unless changed on disk.

4.3 How do I get persistent connections with DBI and CGI?

Contributed by John D. Groenveld

Using Edmund Mergl's Apache::DBI module, database logins are stored in a
hash with each of these "httpd" child. If your application is based on a
single database user, this connection can be started with each child.
Currently, database connections cannot be shared between "httpd" children.

4.4 ``When I run a perl script from the command line, it works, but, when I run it under the httpd, it fails!'' Why?

Basically, a good chance this is occurring is due to the fact that the user
that you ran it from the command line as has a correctly configured set of
environment variables, in the case of DBD::Oracle, variables like
"ORACLE_HOME", "ORACLE_SID" or "TWO_TASK".

The "httpd" process usually runs under the user id of "nobody",
which implies there is no configured environment. Any scripts attempting to
execute in this situation will correctly fail.

One way to solve this problem is to set the environment for your database in a
"BEGIN { }" block at the top of your script. Another technique is to configure
your WWW server to pass-through certain environment variables to your CGI
scripts.

Similarly, you should check your "httpd" error logfile for any clues,
as well as the ``Idiot's Guide To Solving Perl / CGI Problems'' and
``Perl CGI Programming FAQ'' for further information. It is
unlikely the problem is DBI-related.

The ``Idiot's Guide To Solving Perl / CGI Problems'' can be located at:

as can the ``Perl CGI Programming FAQ''. Read BOTH these documents
carefully!

4.5 How do I get the number of rows returned from a SELECT statement?

Count them. Read the DBI docs for the "rows()" method.

Miscellaneous Questions

5.1 Can I do multi-threading with DBI?

Perl version 5.005 and later can be built to support multi-threading.
The DBI, as of version 1.02, does not yet support multi-threading
so it would be unsafe to let more than one thread enter the DBI at
the same time.

It is expected that some future version of the DBI will at least be
thread-safe (but not thread-hot) by automatically blocking threads
intering the DBI while it's already in use.

For some OCI example code for Oracle that has multi-threaded "SELECT"
statements, see:

5.2 How do I handle BLOB data with DBI?

Handling BLOB data with the DBI is very straight-forward. BLOB columns are
specified in a SELECT statement as per normal columns. However, you also
need to specify a maximum BLOB size that the <I>database handle</I> can
fetch using the "LongReadLen" attribute.

would succeed <I>provided no column values were larger than the specified
value</I>.

If the BLOB data is longer than the value of "LongReadLen", then an
error will occur. However, the DBI provides an additional piece of
functionality that will automatically truncate the fetched BLOB to the
size of "LongReadLen" if it is longer. This does not cause an error to
occur, but may make your fetched BLOB data useless.

This behaviour is regulated by the "LongTruncOk" attribute which is
defaultly set to a false value ( thus making overlong BLOB fetches fail ).

### Set BLOB handling such that it's 16Kb and can be truncated
$dbh->{LongReadLen} = 16384;
$dbh->{LongTruncOk} = 1;

Truncation of BLOB data may not be a big deal in cases where the BLOB
contains run-length encoded data, but data containing checksums at the end,
for example, a ZIP file, would be rendered useless.

5.3 How can I invoke stored procedures with DBI?

The DBI does not define a database-independent way of calling stored procedures.

However, most database that support them also provide a way to call
them from SQL statements - and the DBI certainly supports that.

So, assuming that you have created a stored procedure within the target
database, eg, an Oracle database, you can use $dbh->"do()" to
immediately execute the procedure. For example,

$dbh->do( "BEGIN someProcedure; END;" ); # Oracle-specific

You should also be able to "prepare" and "execute", which is
the recommended way if you'll be calling the procedure often.

Remember to perform error checking, though! ( Or use the "RaiseError"
attribute ).

5.5 How can I create or drop a database with DBI?

Database creation and deletion are concepts that are entirely too abstract
to be adequately supported by DBI. For example, Oracle does not support the
concept of dropping a database at all! Also, in Oracle, the database
server essentially is the database, whereas in mSQL, the
server process runs happily without any databases created in it. The
problem is too disparate to attack in a worthwhile way.

Some drivers, therefore, support database creation and deletion through
the private "func()" methods. You should check the documentation for
the drivers you are using to see if they support this mechanism.

5.6 How can I commit or rollback a statement with DBI?

See the "commit()" and "rollback()" methods in the DBI Specification.

Chapter 6 of ``Programming the Perl DBI'' discusses transaction handling within
the context of DBI in more detail.

5.7 How are NULL values handled by DBI?

"NULL" values in DBI are specified to be treated as the value "undef".
"NULL"s can be inserted into databases as "NULL", for example:

$rv = $dbh->do( "INSERT INTO table VALUES( NULL )" );

but when queried back, the "NULL"s should be tested against "undef".
This is standard across all drivers.

5.8 What are these func() methods all about?

The "func()" method is defined within DBI as being an entry point
for database-specific functionality, eg, the ability to create or
drop databases. Invoking these driver-specific methods is simple, for example,
to invoke a "createDatabase" method that has one argument, we would
write:

$rv =$dbh->func( 'argument', 'createDatabase' );

Software developers should note that the "func()" methods are
non-portable between databases.

5.9 Is DBI Year 2000 Compliant?

DBI has no knowledge of understanding of what dates are. Therefore, DBI
itself does not have a Year 2000 problem. Individual drivers may use date
handling code internally and therefore be potentially susceptible to the
Year 2000 problem, but this is unlikely.

You may also wish to read the ``Does Perl have a Year 2000 problem?'' section
of the Perl FAQ at:

Support and Training

The Perl5 Database Interface is FREE software. ITCOMESWITHOUTWARRANTYOFANYKIND. See the DBIREADME for more details.

However, some organizations are providing either technical support or
training programs on DBI. The present author has no knowledge as
to the quality of these services. The links are included for reference
purposes only and should not be regarded as recommendations in any way.
Caveat emptor.

Commercial Support

The Perl Clinic

The Perl Clinic provides commercial support for Perl and Perl
related problems, including the DBI and its drivers. Support is
provided by the company with whom Tim Bunce, author of DBI and
DBD::Oracle, works and ActiveState. For more information on their
services, please see:

Training

Westlake Solutions

A hands-on class for experienced Perl CGI developers that teaches
how to write database-connected CGI scripts using Perl and DBI.pm. This
course, along with four other courses on CGI scripting with Perl, is
taught in Washington, DC; Arlington, Virginia; and on-site worldwide upon
request.

AUTHOR

COPYRIGHT

This document is Copyright (c)1994-2000 Alligator Descartes, with portions
Copyright (c)1994-2000 their original authors. This module is released under
the 'Artistic' license which you can find in the perl distribution.

This document is Copyright (c)1997-2000 Alligator Descartes. All rights reserved.
Permission to distribute this document, in full or in part, via email,
Usenet, ftp archives or http is granted providing that no charges are involved,
reasonable attempt is made to use the most current version and all credits
and copyright notices are retained ( the AUTHOR and COPYRIGHT sections ).
Requests for other distribution rights, including incorporation into
commercial products, such as books, magazine articles or CD-ROMs should be
made to Alligator Descartes <http://www.symbolstone.org/descarte/contact.html>.